What I gain from increasing my aperture from f2.0 to f1.4 is minimal, especially with the ever-better noise levels at high iso levels. I also like the fact that the MP can get that bit closer... 24cm instead of 45cm... I very rarely get that close but it is very handy when sitting across from a portrait subject in cafes for example. The bokeh from both is excellent, of course f1.4 is better than f2.0... but throw it in closer to the subject and the effect is stronger... although it is not that flattering to get too close to subjects with a 50mm, but it does give another perspective....The f1.4 is also around 190g lighter if you are considering lugging kit up mountains, it may well help...

The 100mm MP gets more use with me for head & shoulders or the jewellery images I am lumbered with... it is the single most difficult product I know to photograph... and bugs the hell out of me... but it does pay the bills...

Anyway, the initial poster may be well serviced by visiting the Official Carl Zeiss Lenses Group, on Flickr: